1. Field of the Invention
The present invention is directed to a process for selectively removing alcohols, especially C.sub.1 -C.sub.10, preferably C.sub.1 -C.sub.4 most especially methanol from solutions comprising alcohol, ether, olefin and non-linear hydrocarbons, or alcohol/ether or alcohol/olefin, or alcohol/nonlinear hydrocarbons by use of membranes and the removal of ethers from mixed ether hydrocarbon streams. The solutions of alcohol, especially solutions of alcohols, ether, olefins, and non-linear hydrocarbons are typically the synthesis solutions encountered in the production of high value ethers such as methyl tertiary-butyl ether (MTBE) and tertiary-amyl methyl ether (TAME) used as octane enhancers in motor fuels. The alcohol is present as one of the starting materials in the ether synthesis but cannot readily be separated from the synthesis solution due to azeotrope formation.
The present invention selectively removes the alcohols from solutions comprising alcohol/ether, alcohol/non-linear hydrocarbons alcohol/olefins or alcohol/ether/olefin/non-linear hydrocarbons by selectively permeating the alcohol under pervaporation conditions through polyester and polyester copolymer membranes, or selectively removes ether from an ether-hydrocarbon stream by selective permeation of the ether through the membrane, again under pervaporation conditions.
2. Description of the Related Art
Environmental concerns have focused on reducing pollution associated with motor vehicles, especially gasoline burning internal combustion engine vehicles. While improvements in vehicle and engine design can account for part of the desired improvements in emission levels, much attention has been focused on improvements in the motor vehicle fuels, especially gasoline. Legislation is either being considered or being actively pursued on the state and federal level to mandate lower allowable aromatics and sulfur content in gasoline and reduce the final boiling point and alter the vapor pressure of gasolines. Much as when lead was phased out of gasoline it is anticipated that these changes in motor gasoline formulation will inevitably become world wide.
Lowering the emissions of fuels by removing lead octane additives and reducing aromatics contents however, have forced refiners to seek new, environmentally acceptable octane enhancers to make up the loses due to regulation.
Oxygen containing organic molecules such as ethers, in particular methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME) are very high octane components which are currently viewed as environmentally acceptable and the additives of choice to make up the octane shortfall. As a result, demand for these materials has increased as has their price.
MTBE and TAME and other ethers and mixtures thereof are produced by any of numerous processes which utilize methanol or other alcohols and hydrocarbons in the synthesis.
To be economically attractive these processes must convert the hydrocarbon, e.g., isobutylene or isoamylene to as high a degree as possible to ether, e.g., MTBE or TAME respectively. To accomplish this the alcohol, e.g., methanol is added in excess to shift the equilibrium as far as possible to product formation. Excess alcohol is also desirable in that an increase in catalyst life due to reduced polymerization is also achieved.
A problem is encountered however when one seeks to separate the unreacted alcohol from the ether and hydrocarbon for recycle and product purification. Alcohol forms an azeotrope with the ether/hydrocarbon mixture and cannot be readily separated by distillation.
Membrane separation processes are not affected by azeotrope formation. However, membrane processes typically do not achieve the high purities usually associated with distillation.